1. Field of the Invention
This invention relates to methods for fabricating an electrical conductive structure of a circuit board, and more particularly, to a method for fabricating an electrical conductive structure, which can be applied to the electrical conduction between two different electrically conductive pads and meets the requirement of fine pitch.
2. Description of Related Art
With the rapid development of electronic industry, modern electronic devices are designed to have compact size, high performance, multiple functions and fast operating speed. A flip chip semiconductor packaging technique is one of the most advanced semiconductor packaging techniques in the art. The flip chip semiconductor packaging technique includes forming solder bumps on the electrode pads of an integrated circuit (IC) chip, and forming solder bumps on a packaging substrate electrically conductive pads and the solder bumps are corresponding in position to each other, so as to dispose on the packaging substrate the IC chip with the active surface of the IC chip facing down.
Because more and more electric devices are required to have compact size, multiple functions and fast operating speed, the flip chip semiconductor packaging technique is widely applied to various fields, and has become a standard chip packaging technique. Moreover, in order to improve the electricity performance of the electric devices, varieties of passive components, such as resistors, capacitors and inductors, are disposed in the electric devices. The surface mounting devices such as the passive components are disposed on circuit boards by a surface mounting technology (SMT). There are a plurality of solder bumps and surface-mounting metal conductive components disposes on the circuit boards, solder materials of the solder bumps and surface-mounting metal conductive components having different sizes and heights for disposed different kind device. Therefore, the IC chip is electrical conductive to the solder bumps and the surface mounting device is electrical conductive to surface-mounting metal conductive components.
Chemical deposition and stencil printing technologies are two of the most popular techniques adopted in the art to form the solder materials on a substrate. As shown in FIG. 1A, an organic insulating protection layer 21, such as a solder mask, covers a circuit board 20 having a plurality of electrically conductive pads 201. The insulating protection layer 21 is formed by a patterning process and comprises a plurality of openings 211 for exposing the electrically conductive pads 201. As shown in FIG. 1B, a metal adhesive layer 22 is further formed on the electrically conductive pads 201 by sputtering, evaporating or electroless plating (or called chemical deposition processes). As shown in FIGS. 1C and 1D, a stencil 23 having a grid 23a is used to print on the metal adhesive layers 22 on part of the electrically conductive pads 201 of the circuit board 20 and form a solder material 24. As shown in FIG. 1E, a solder bump 24′ is formed by the use of a re-flow process. Therefore, two different materials, i.e. the solder bump 24′ and metal adhesive layers 22, are formed on the electrically conductive pads 201.
However, the higher the solder material 24 or solder bump 24′ formed by the printing of the mold board 23 is, the more difficult the controlling of the height in succeeding processes becomes. In consequence, the re-flowed bumps and solder balls are not equal in height, and their electrical conduction to a chip or a printing circuit board is severely impacted. Moreover, the formation of the bumps and solder balls by printing the solder material on the electrically conductive pads of the circuit board easily causes too many solder materials to be melt, resulting in a bridging phenomenon (solder materials joined together at neighboring electrically conductive pads) and short problem and the failing of providing electrically conductive pads of fine pitches. Further, a great deal of the use of solder materials harms the environment.
Therefore, how to solve the above drawbacks is becoming one of the most urgent issues in the art.